Libya's civil war was poised to resume as an ultimatum to two key cities to surrender to rebel forces ran out.

Last night, rebel troops were on their way to reinforce front lines near Bani Walid, south of Tripoli where Saif al-Islam and Mutassim Gaddafi -- two of the former dictator's sons -- were said to be leading last-ditch defences.

Leaders of the rebel National Transitional Council (NTC) have been negotiating with tribal leaders in the town to surrender peacefully. But sources have revealed that that a refusal to offer immunity to local Gaddafi volunteers accused of crimes carried out in his name stood in the way of an agreement.

In any case, the 500 to 600 Gaddafi fighters the rebels believed to be in the town would not be bound by an agreement made by local leaders -- and Saif al-Islam, along with his father, has issued public statements promising to fight on. After the last Gaddafi forces fled Tripoli last weekend, led by Gaddafi and at least two of his children who were all spotted in southern Tripoli on Friday of last week, fighting halted.

Ultimatum

The NTC gave the last three major Gaddafi strongholds -- Bani Walid, Sirte on the coast, and Sabha in the southern desert -- until today to surrender. The ultimatum to Sirte, where a major battle has been expected for weeks, was extended to give local leaders more time to persuade their followers to change sides. That did not apply to Bani Walid, Dr Mahmoud Abdul Aziz, NTC member for the town, said. He believed the rebel forces would be welcomed into the town, adding: "Believe me, the people there are ready to receive us, and we will attack very soon."

On Thursday, Colonel Abdulrazzaq al-Nadouli, deputy commander of rebel forces in the town of Tarhouna, near the front line, said his troops were just 40 miles from Bani Walid.